High-resolution digital input imaging processes are desirable for superior quality printing applications, especially high quality color printing applications. As is well known, such processes may include electrostatographic processes using small-particle dry toners, e.g., having particle diameters less than about 7 micrometers, electrostatographic processes using solvent based liquid developers in which the particle size is typically on the order of 0.1 micrometer or less, and ink-jet processes using aqueous or solvent based inks.
The most widely used high-resolution digital commercial electrostatographic processes involve electrophotography. Although capable of high process speeds and excellent print quality, electrophotographic processes using dry or liquid toners are inherently complicated, and require expensive, large, complex equipment. Moreover, due to their complex nature, electrophotographic processes and machines tend to require significant maintenance.
Ink jet technology may be used to deposit fluid materials on substrates and has numerous applications, mainly in printing. Ink jet printers function by depositing small droplets of fluid at desired positions on a substrate. There are various ink jet printing technologies.
Typical ink jet inks suffer from several problems that limit their usefulness in commercial printing. Inks are generally dye based and subject to fading. The inks have to have low viscosity to minimize plugging of the ink jet nozzles. As a result, the ink must be jetted onto specially formulated absorbent papers. The absorbency of the drops results in low image density and image spread. Clay coated graphic arts papers commonly used in the printing industry to print high quality images cannot be used in ink jet engines because the ink would run on the nonabsorbent clay coated paper. Rather, for high quality imaging, expensive and limited types of papers are used with ink jet printing.
Another limitation to using ink jet technology for commercial printing applications is that all ink has to be dried. This requires a lot of energy to evaporate the water or organic solvents used in the ink and generates a lot of moisture and/or solvent vapors.
To overcome problems associated with fading and low image densities associated with dyed aqueous-based inks, pigmented aqueous-based inks have been disclosed in which a pigmented material is colloidally dispersed. Typically, a relatively high concentration of pigmented material is required to produce the desired highest image densities (Dmax). Exemplary art pertaining to pigmented aqueous-based inks includes U.S. Pat. No. 6,143,807 (Lin et al.) and U.S. Pat. No. 6,153,000 (Erdtmann et al.). Generally, pigmented inks have a much greater propensity to clog or alter the jet(s) orifice(s) of a drop-on-demand type of ink jet head than do dyed inks, especially for the narrow diameter jets required for high resolution drop-on-demand ink jet imaging, e.g., at 600 dots per inch. Drop-on-demand printers do not have a continuous high pressure in the nozzle, and modification of the nozzle behavior by deposition of pigment particles is strongly dependent on local conditions in the nozzle. In continuous ink jet printers using pigmented inks, the relatively high concentrations of pigment typically affects the droplet break-up, which tends to result in non-uniform printing.
In co-pending U.S. patent application Ser. Nos. 11/445,712; 11/446,467; 11/445,713; 11/445,714; 11/445,566, an apparatus, a method, and an ink that can be used to produce high quality at production rates suitable for use in a graphic arts establishment are disclosed. This can be described in brief. Ink having electrically charged marking particles in a dielectric solvent is jetted onto an electrically conducting primary imaging member. The inked image is then passed by a fractionator that subjects the ink to an electric field that drives the marking particles towards the primary imaging member and skives the supernatant solvent off the image, thereby leaving a concentrated ink in the areas corresponding to the image. The charged particles are then pressed into a nip formed by the convergence of the primary imaging member and a receiver. The receiver could be a final image receiver such as paper that is pressed against a roller located on the non-image receiving side of the paper, etc. Alternatively, the receiver could be a transfer intermediate which would receive the image at this point and subsequently transfer it from the intermediate to a final receiver. The image is then transferred from the primary imaging member to the receiver by applying an electrostatic field of such magnitude and polarity as to drive the marking particles towards the receiver.
By transferring concentrated ink, the ink neither runs nor soaks into the paper to any significant extent. This allows one to achieve a high-density image, minimize image spread, and also allows the use of clay-coated papers. Moreover, the amount of solvent that has to then be removed has been greatly reduced, thereby reducing energy consumption and minimizing vapor emissions.
There is prior art that addresses the problem of having excessive fluid in the image by forming the image on an intermediate, then transferring the image to a receiver. U.S. Pat. No. 5,099,256 discloses the use of cylinder specifically coated with a silicone polymeric material in combination with a drop on demand print head. U.S. Pat. No. 6,736,500 discloses the use of a coagulating agent that increases the viscosity of the ink jet ink and improve transfer and image durability. U.S. Pat. Nos. 6,755,519 and 6,409,331 teach methods for increasing ink viscosity such as via UV cross-linking or evaporation. None of these patents address the formation of a multi-color image.
U.S. Pat. Nos. 6,761,446; 6,767,092; 6,719,423; and 6,761,446 refer to forming images on separate intermediates, then transferring the images in register to form a four-color image on a receiver. While these patents address the problem of excess fluid in a four-color image, the process of registration of the component images from separate intermediates involve complex and expensive mechanisms. The situation is further complicated if receivers of different thickness and/or surface properties need to be used. In addition, the receiver path to accommodate successive transfers to form the multi-color image is relatively long, affecting cost and reliability.
A major difficulty encountered with the ink jet developers that use organic solvents is that they tend to spread. This would result in image degradation problems such as a lack of sharpness and resolution. To mitigate this problem, the use of a cellular primary imaging member has been proposed in co-pending U.S. patent application Ser. No. 11/446,467.
Cellular structures are commonly used in printing. For example, in gravure printing, a gravure roller or plate is used. The roller or plate includes a cellular pattern that corresponds to the image to be printed, with the size of each cell corresponding to the amount of ink required to produce a certain density. The gravure roller or plate is produced for each specific image to be printed. It should also be noted that only approximately half of the ink transfers to a receiving member in gravure printing, with the rest remaining in the cells. The residual ink in the cells does not create a problem for traditional gravure printing because identical images are produced in register with the gravure roller or plate. This would, however, create problems for digital printing, where the images may not be produced in register and may, in fact, differ from each other.
Another type of cellular member commonly used in the printing industry is the anilox roller. This device has a periodic array of uniformly sized cells that are used to apply ink in a non-image-wise fashion to an inking member.
A cellular structure can be used as a primary imaging member to minimize the spread of ink droplets jetted from an ink jet nozzle. However, unlike a conventional gravure roller or plate that is job specific, it is highly preferable to use a universal primary imaging roller or plate that can be used for many different images. Such primary imaging roller or plate is capable of having all marking material removed with the printing of each image so that the primary imaging member can be effectively used in digital printing applications without undue stress applied to a cleaning subsystem that would have to remove all residual ink.
To use a cellular or textured imaging member, hereafter referred to as a TIM as a primary imaging member in an ink jet printing press and to minimize the spread of the ink droplets, the ink droplets must be injected into each cell to form a positive image, with the gray scale controlled by varying the amount of ink injected into a cell. The varying amounts of ink in the uniformly sized cells creates a problem transferring the ink because the receiver does not contact the ink.